1. Field
This disclosure relates generally to charge pump systems, and more specifically, to charge pump systems with a switchable impedance.
2. Related Art
Charge pumps are commonly used in integrated circuit applications to provide a regulated DC voltage source. FIG. 1 illustrates a charge pump system in accordance with the prior art which, when enabled, provides Vout at a desired target voltage. Vout is provided to a voltage divider formed by resistors 12 and 16, in which an output 14 of the voltage divider is provided to a negative input of a comparator 18. A reference voltage, Vref, is provided to the positive input of comparator 18. Vref is selected based on the desired target voltage for Vout. The output of comparator 18 is provided to clocks and charge pump unit 20 which provides Vout. Clocks and charge pump unit 20 uses switching devices and capacitive elements to adjust Vout based on the output of comparator 18. For example, when the voltage at node 14, which is a fraction of Vout, is below Vref, the output of comparator 18 is at a first logic state which enables clocks and charge pump unit 20. This results in unit 20 increasing Vout to the target voltage level. Upon the voltage at node 14 reaching Vref, the output of comparator 18 changes to a second logic state and disables clocks and charge pump unit 20. At this point, due to the load coupled to Vout, Vout decreases which causes the voltage at node 14 to decrease. However, once the voltage at node 14 again falls below Vref, comparator 18 toggles back to the first logic state, thus re-enabling clock and charge pump unit 20. Note that any known configuration may be used for clocks and charge pump unit 20. In system 10 of FIG. 1, large ripple results on Vout resulting from the delay through comparator 18 due to the attenuation of the differential input to the comparator by the resistor divider feedback formed by resistors 12 and 16. This creates undesirable noise in Vout.
FIG. 2 illustrates another prior art charge pump system configuration which attempts to address the ripple resulting from the voltage divider of FIG. 1. Charge pump system 21 of FIG. 2 includes a capacitor 25 coupled between Vout and node 24. Capacitor 25 provides an AC bypass of the voltage divider including resistors 22 and 23. Capacitor 25 decreases the steady state ripple on Vout by increasing the differential input (at node 24) to the negative input of comparator 26 during steady state operation of system 21. However, capacitor 25 creates an RC delay in reaching the desired DC operating point (the desired target voltage) during the startup transition of charge pump system 21. Therefore, ripple on Vout is reduced at the expense of increasing start up time.
Therefore, a need exists for an improved charge pump system which addresses the issues of ripple and start up time.